Re: gdb / thread_db / multiple ABI question

[Pedro Alves <palves@redhat.com>]

On 07/07/2017 07:20 PM, Steve Ellcey wrote:

> It makes sense that the LP64 gdb cannot dlopen the ILP32 thread_db that
> corresponds to the ILP32 thread library that the program being debugged
> uses but I don't know what to do about it.
> 
> I looked at x86 and when I compile a 32 bit program there and then debug
> it with a 64 bit gdb I see:
> 
> 	[Thread debugging using libthread_db enabled]
> 	Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
> 
> So I am guessing that aarch64 needs to use the 64 bit libthread_db.so.1 even
> when debugging 32 bit programs, is that right?  

Yes.  

Though I have no idea whether an Aarch64 build of libthread_db is
capable of biarch debugging.  It may be that GDB is already trying to
the load the 64-bit libthread_db, but then libthread_db rejects
the inferior for some reason.  The "set debug libthread-db" option should
help you figure that out.  See below.

> What do I need to do to tell
> gdb to do that?  I tried adding /home/sellcey/gdb-ilp32/install/lib64 to
> solib-search-path but that did not help.  I am not sure what triggers the
> difference in behaviour between aarch64 and x86.

See:

(gdb) apropos libthread
info auto-load libthread-db -- Print the list of loaded inferior specific libthread_db
set auto-load libthread-db -- Enable or disable auto-loading of inferior specific libthread_db
set debug libthread-db -- Set libthread-db debugging
set libthread-db-search-path -- Set search path for libthread_db
show auto-load libthread-db -- Show whether auto-loading inferior specific libthread_db is enabled
show debug libthread-db -- Show libthread-db debugging
show libthread-db-search-path -- Show the current search path or libthread_db

The related GDB code is in gdb/linux-thread-db.c.

Thanks,
Pedro Alves